The present invention generally relates to providing variable electrical power to a three phase electrical motor. More specifically, the present invention relates to varying electrical power to a three phase electrical motor based on varying frequency input turning an electrical generator which provides electrical power to the motor, in order to overcome variable conditions encountered by the motor that require a variation in electrical power to the motor.
It is known to use three phase generators to supply the power to drive three phase motors. Normally, fixed power supply systems using three phase generators are operated at a constant speed to deliver three phase electrical energy of constant frequency to supply three phase power to three phase motors. There are currently two types of three phase fixed power supply systems in common use today. They are the 50 hertz and the 60 hertz power supply systems, which are used to drive electrical motors rated for 50 or 60 hertz operation, respectively. A problem with a single generator to supply power to a single motor system is that the large starting current required by the motor is normally overcome by sizing the generator as much as 40 percent larger than the main motor load to be applied to the generator. If the motor-generator combination is less than 40 percent larger than the main electrical motor to be started, then the engine driving the generator usually stalls out when attempting to start the motor. Another problem with fixed power supply systems is that these systems can not vary the frequency of the output power to the three phase motor, if it is required.
Some applications of three phase motors require a variable frequency output power to the motor, due to varying operational conditions required of the motor. An example is electrical submersible pumps used in the oil and gas industry at remote pumping stations. Typically, these pumps are located where there is an inadequate local electrical power grid supply or even no power grid supply and it is a common practice to use an engine to power the generator on the well site to supply power to the electric submersible pump motor. The engines are usually diesel, gasoline, natural gas, or propane powered. It is known that for any given centrifugal pump the rate and head capability of that pump is greatly expanded through the use of variable speed operation which is accomplished by providing power in the form of variable frequency power to the motor of the pump. This allows a pump to fit a larger process application due to varying conditions during the time that the pump is installed. In the oil and gas industry, a pump can be installed in a well for up to three or four years before being worn out and requiring a replacement. During this time, the production from an oil or gas well can change for a number of reasons due to varying conditions, the following are some examples. There is a decline in reservoir pressure due to depletion of the well. There is an increase in reservoir pressure due to voidage replacement from injection wells. There are changes in fluid produced, as usually oil is decreasing and water is increasing in a well. There could also be an increase or decrease in gas produced in the well, which would affect the fluid production of the well. There are mechanical problems with the wellbore, such as scaling or paraffin etc., which could cause a production decline. Chemicals can be used to remove these mechanical problems that restrict production without removing the pump, which will result in a production increase. These varying fluid production conditions can be accommodated by the varying of the power supply frequency to the motor of the pump to maximize oil production from wells.
When it is desired to operate a three phase motor at a variable speed the current method is to input the constant frequency three phase power from a generator to a three phase variable speed drive. Sometimes the variable speed drive is referred to as a variable frequency drive. The variable speed drive supplies three phase power at a variable frequency to the three phase motor. The variable speed drive allows the three phase motor to be operated at a variable speed in direct proportion to the variable frequency of the power supply from the variable speed drive. Sometimes a transformer is required between the variable speed drive and the three phase motor to provide a voltage match required by the three phase motor. FIG. 1 shows an example of currently used systems for generating three phase variable frequency power to a three phase motor. The system includes an engine, three phase electrical generator, excitation controller, variable speed drive, transformer, switchboard, motor controller and the motor. The engine speed is controlled by a governor to maintain constant engine speed under varying load conditions imposed by the generator load and a fifty or sixty Hertz output requirement. The engine is usually mechanically coupled to the electrical generator, but other connections are also used. The generator voltage output is controlled by the excitation controller. The output voltage of the generator is electrically connected by cables to the variable speed drive. The output of the variable speed drive is electrically connected by cables to the transformer. The transformer is normally of a multi-tap configuration to provide the appropriate voltage required by the motor. The multi-tap allows for numerous motor combinations to be used, which also makes it easier to accommodate the cable voltage loss associated with the wide range of cable sizes and lengths required to reach electric submersible pumps installed deep in oil & gas wells. A switchboard used as a power disconnect is connected along the cables between the transformer and the motor. A motor controller is an electronic unit used to control the frequency output of the variable speed drive as required by the motor based on feedback of varying conditions encountered by the motor. The motor controller also controls the on-off function of the switchboard. The motor controller is connected by cables to the switchboard, and by cables to the variable speed drive to monitor, control, and adjust the power to the motor by numerous programmed parameters.
There are many disadvantages to using a variable speed drive. Use of a variable speed drive reduces the electrical efficiency of the motor and creates harmonic spikes on the generated variable frequency power supplied to the motor. A variable speed drive reduces the power factor of the motor. A variable speed drive increases the waste heat generated in all the electrical equipment and wiring on the output side of the variable speed drive, including the transformer, wiring and the motor. A variable speed drive also requires more frequent maintenance compared to the rest of the electrical equipment in the system. Other disadvantages are associated with the engine, generator and transformer, when using a variable speed drive. The engine needs to be sized at least twenty percent larger than the motor load to accommodate the high electrical losses in the system. The transformer must have a larger number of multiple voltage taps to anticipate changes in motor and cable requirements, which requires a more expensive transformer. The transformer core also needs to be about twenty percent more massive than standard transformers to handle the harmonics and low frequency operation. The transformer also needs higher insulation rating on the wiring to handle the high voltage spikes due to harmonics. The generator also needs to be about twenty percent more massive than standard generators to handle the harmonics and low frequency operation. The generator also needs higher insulation rating on the wiring to handle the high voltage spikes due to harmonics.
It is an object of the present invention to provide a system and method to vary the electrical power to a three phase motor without the use of a variable speed drive between the generator and the motor.